Platens are essentially robustly constructed support structures that locate, support and align mold halves under applied clamp/closure tonnage. Each platen in a system must therefore be arranged to convey force effectively to the mold. In a molding process, as will be understood, it is important to maintain a high degree of parallelism between surfaces of a platen (and also between platens), since distortion in the platen correspondingly and usually induces distortion in a mold half (and particularly the mold face) located within the platen. Indeed, under applied clamp tonnage, platens should ideally be entirely resilient to physical distortions in their structure, thereby ensuring that a mold surface remains undistorted and hence substantially (and preferably entirely) flat. It is important to maintain, as far as possible, the flatness of the mold and platens.
In use, mold-bearing faces of co-operating platens are moved relative to each other to cause the formation of a mold through the aligned abutment of complementary mold halves. Under subsequently applied clamp tonnage, injection of melt can commence into a resultant mold cavity defined by the two mold halves. Once in abutting and locked engagement, a clamp force is conveyed through the platens. Molten plastics material may then be injected (by an injection unit) into a mold cavity defined by the mold halves, thereby to form an article having a predefined shape. To enhance productivity, the mold halves are cooled by a water cooling system comprising a number of water-carrying tubes, which system increases the rate at which the injected (e.g. molten plastics material) solidifies. The clamping force is then removed and the mold halves opened/separated to allow ejection or extraction of the molded article. The process can then be repeated.
Machines for injection molding of plastics articles may include a pair of fixed platens that are spaced from each other and that are interconnected by generally four parallel tie bars that have their axes positioned to define a rectangular array. One of the fixed platens remains stationary and is adapted to support one portion of a two- or multiple-piece injection mold that when assembled or engaged defines a mold cavity to correspond with the outline of a desired molded part. A movable platen is slidably carried on the tie bars between the fixed platens and is adapted to carry a cooperating portion of the mold so that when the movable platen is moved toward the mold-portion-carrying fixed platen, the two mold portions come into contact to define therebetween a mold cavity for forming the desired part.
The movable platen is generally a plate-like structure that is of rectangular configuration and includes four bores at the respective corners, through each of which a tie bar extends. A movable platen actuation system is positioned between the non-mold-carrying fixed platen and the movable platen to cause the movable platen to move along the tie bars toward or away from the mold platen, and also to hold the movable platen firmly in position when the mold portions are together, to prevent separation of the molds as molten material is injected into the mold cavity under high pressure.
Injection molding machine platens are typically block-shaped as shown in U.S. Pat. No. 5,188,850 to Hirata et al.; U.S. Pat. No. 5,066,217 to Fukuzawa et al.; U.S. Pat. No. 5,110,283 to Bluml et al.; U.S. Pat. No. 5,123,834 to Joyner; U.S. Pat. No. 5,162,782 to Yoshioka; U.S. Pat. No. 5,192,557 to Hirata et al; U.S. Pat. No. 5,593,711 to Glaesener; and U.S. Pat. No. 5,776,402 to Glaesener. In each of these patents, the mold platen is substantially block-shaped, having a substantially rectangular side which supports a mold half. During clamp-up of mold halves, a force is generated against the mold-mounting face of the platen. As is typical with such block-shaped platens, the mold-mounting face is caused to concavely deform, forcing the top and bottom edges of the side towards the oncoming force and causing the platen to bend and provide tension across the back side of the mold platen. As a result, under the clamp-up force, the center of the platens separate causing a gap between the mold halves, and in some cases, the formation of flash is a by-product.
U.S. Pat. No. 4,615,857 to Baird discloses an encapsulation means and method for reducing flash during mold operations. In accordance with this device, it is alleged that injection and transfer molding of plastic is performed in a manner that virtually eliminates flash. Deflection of the mold press therein is measured with the mold press in the clamping configuration. The support structure of the mold is arranged to apply an equal force to the mold face by configuring supporting pillars and bars such that they act as individual springs against the mold face and in response to the force generated while the mold is in the clamping configuration. The spring constants and the lengths of the supporting pillars and bars are calculated to account for the actual deflection found in the mold press, thereby producing a uniform pressure on the molds during clamping of the press.
U.S. Pat. No. 6,439,876 to Glaesener discloses an injection molding which includes a stationary platen having a first mold half and at least one movable platen having a second mold half for forming a mold with said first mold half. For one of the platens, a back face is positioned central to the platen for the connection to a pressure source such as a hydraulic cylinder. Parallel to and spaced from the back face is a front face for carrying a mold half, and protruding rearward from the periphery of the front face is a structural wall. Extending from the back face to the front face are a plurality of spaced ribs which are rigidly affixed to the structural wall in a structurally-optimized pattern. Means for uniformly directing the clamping force from the center of the back face to the front face to reduce localized bending of the mold face is provided.
While the platens described in the prior art help to properly distribute forces to the mold, each platen is designed to optimize the distribution of forces with respect to a particular mold size. If platens of incorrect size are used, the mold halves are subject to bending, as described above, which results in unwanted flash on the molded product. Consequently, if a larger or smaller mold is used, the platens must be removed and replaced with a different size platen to ensure that the forces will be distributed properly over the entire mold half. This requires that the molding machine be taken offline while the platen is removed and replaced with a platen of appropriate size.
Accordingly, there exists a need for an adjustable platen which can be used with molds of different sizes and which includes means for eliminating or reducing platen deflection during mold or press clamp-up and which substantially eliminates the creation of flash during injection molding.